In satellite communication systems, microwave signals are beamed between one or more ground stations and a satellite, or from one satellite to another satellite. In the ground station to satellite communication systems, each beam of signals is controlled to ensure a specific area of coverage, for example, a wide beam may cover a large area such as a country or continent, while a narrow beam may cover a small area such as a specific ground station. Additionally, a plurality of beams may be simultaneously radiated to cover a plurality of independent coverage areas. This technique is commonly referred to as multiple beam coverage. The present invention addresses the generation and positioning of independent multiple beams.
Networks which include multiple beam phased array antennas and devices for transmitting and receiving microwave signals are known for satellite communication systems, as evidenced by, for example, U.S. Pat. No. 3,953,857, issued Apr. 27, 1976, entitled "Airborne Multi-mode Radiating and Receiving System", by F. Jenks. Other examples include U.S. Pat. No. 4,521,781, issued Jun. 4, 1985, entitled "Phase Scanned Microstrip Array Antenna", by Campi et. al.; U.S. Pat. No. 4,652,880, issued Mar. 24, 1987, entitled "Antenna Feed Network", by Moeller et. al.; U.S. Pat. No. 4,734,700, issued Mar. 29, 1988, entitled "Group Antenna with Electronically Phased-Controlled Beam", by Brunner; U.S. Pat. No. 4,766,438, issued Aug. 23, 1988, entitled "Three Dimensional Feed Through Lens with Hemispherical Coverage", by Tang; and U.S. Pat. No. 4,799,065, issued Jan. 17, 1989, entitled "Reconfigurable Beam Antenna", by Thompson.
The conventional multiple beam phased array antennas and beam forming networks employ machined or electro-formed horns, separate filters, and delay line or ferrite phase shifters. These devices are coupled to wave guides and coaxial transmission lines, as well as other microwave components. These conventional configurations are relatively large and heavy. Large and heavy antennas are a disadvantage because the antennas are typically deployed in spacecraft where increased size and weight lead to increased launch costs. The conventional phased array antennas are also difficult and expensive to implement on a recurring basis because their components, the horns, filters and phase shifters, are individual electrical devices whose characteristics may vary from device to device. Additionally, it is difficult and expensive to assemble these devices into antennas in a manner which ensures that uniform antenna characteristics are maintained throughout the array.
Recently, the number of satellites deployed in geosynchronous orbit about the earth has increased significantly. The increase in the number of deployed satellites has lead to an increase in the number of microwave signals being transmitted from both the deployed satellites and communication networks based on the surface of the earth. As a result, higher interference levels are being experienced as the deployed satellites and the earth based networks attempt to communicate.
Thus, there remains a need for an efficient, light weight, easy to implement satellite communication system that minimizes interference from other systems.